12 CHAPTER OUTLINE 12.1 The Conditions for Equilibrium 12.2 More on the Center of Gravity 12.3 Examples of Rigid Objects in Static Equilibrium 12.4 Elastic Properties of Solids Static Equilibrium and Elasticity ANSWERS TO QUESTIONS Q12.1 When you bend over, your center of gravity shifts forward. Once your CG is no longer over your feet, gravity contributes to a nonzero net torque on your body and you begin to rotate. Q12.2 Yes, it can. Consider an object on a spring oscillating back and forth. In the center of the motion both the sum of the torques and the sum of the forces acting on the object are (separately) zero. Again, a meteoroid flying freely through interstellar space feels essentially no forces and keeps moving with constant velocity. Q12.3 No—one condition for equilibrium is that F ∑ = 0 . For this to be true with only a single force acting on an object, that force would have to be of zero magnitude; so really no forces act on that object. Q12.4
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